daddy/internal/mail/send.go

package mail

import (
	"crypto/tls"
	"fmt"
	"math/rand"
	"net/mail"
	"strconv"
	"strings"
	"time"

	"github.com/pkg/errors"
	gomail "gopkg.in/mail.v2"
)

var (
	ErrUnexpectedEmailAddressFormat = errors.New("unexpected email address format")
)

type SendFunc func(*SendOption)

type SendOption struct {
	Charset           string
	AddressHeaders    []AddressHeader
	Headers           []Header
	Body              Body
	AlternativeBodies []Body
}

type AddressHeader struct {
	Field   string
	Address string
	Name    string
}

type Header struct {
	Field  string
	Values []string
}

type Body struct {
	Type        string
	Content     string
	PartSetting gomail.PartSetting
}

func WithCharset(charset string) func(*SendOption) {
	return func(opt *SendOption) {
		opt.Charset = charset
	}
}

func WithSender(address string, name string) func(*SendOption) {
	return WithAddressHeader("From", address, name)
}

func WithSubject(subject string) func(*SendOption) {
	return WithHeader("Subject", subject)
}

func WithAddressHeader(field, address, name string) func(*SendOption) {
	return func(opt *SendOption) {
		opt.AddressHeaders = append(opt.AddressHeaders, AddressHeader{field, address, name})
	}
}

func WithHeader(field string, values ...string) func(*SendOption) {
	return func(opt *SendOption) {
		opt.Headers = append(opt.Headers, Header{field, values})
	}
}

func WithRecipients(addresses ...string) func(*SendOption) {
	return WithHeader("To", addresses...)
}

func WithCopies(addresses ...string) func(*SendOption) {
	return WithHeader("Cc", addresses...)
}

func WithInvisibleCopies(addresses ...string) func(*SendOption) {
	return WithHeader("Cci", addresses...)
}

func WithBody(contentType string, content string, setting gomail.PartSetting) func(*SendOption) {
	return func(opt *SendOption) {
		if setting == nil {
			setting = gomail.SetPartEncoding(gomail.Unencoded)
		}
		opt.Body = Body{contentType, content, setting}
	}
}

func WithAlternativeBody(contentType string, content string, setting gomail.PartSetting) func(*SendOption) {
	return func(opt *SendOption) {
		if setting == nil {
			setting = gomail.SetPartEncoding(gomail.Unencoded)
		}
		opt.AlternativeBodies = append(opt.AlternativeBodies, Body{contentType, content, setting})
	}
}

func (m *Mailer) Send(funcs ...SendFunc) error {
	opt := &SendOption{
		Charset: "UTF-8",
		Body: Body{
			Type:        "text/plain",
			Content:     "",
			PartSetting: gomail.SetPartEncoding(gomail.Unencoded),
		},
		AddressHeaders:    make([]AddressHeader, 0),
		Headers:           make([]Header, 0),
		AlternativeBodies: make([]Body, 0),
	}

	for _, f := range funcs {
		f(opt)
	}

	conn, err := m.openConnection()
	if err != nil {
		return errors.Wrap(err, "could not open connection")
	}

	defer conn.Close()

	message := gomail.NewMessage(gomail.SetCharset(opt.Charset))

	for _, h := range opt.AddressHeaders {
		message.SetAddressHeader(h.Field, h.Address, h.Name)
	}

	for _, h := range opt.Headers {
		message.SetHeader(h.Field, h.Values...)
	}

	froms := message.GetHeader("From")

	var sendDomain string

	if len(froms) > 0 {
		sendDomain, err = extractEmailDomain(froms[0])
		if err != nil {
			return err
		}
	}

	messageID := generateMessageID(sendDomain)
	message.SetHeader("Message-Id", messageID)

	message.SetBody(opt.Body.Type, opt.Body.Content, opt.Body.PartSetting)

	for _, b := range opt.AlternativeBodies {
		message.AddAlternative(b.Type, b.Content, b.PartSetting)
	}

	if err := gomail.Send(conn, message); err != nil {
		return errors.Wrap(err, "could not send message")
	}

	return nil
}

func (m *Mailer) openConnection() (gomail.SendCloser, error) {
	dialer := gomail.NewDialer(
		m.opt.Host,
		m.opt.Port,
		m.opt.User,
		m.opt.Password,
	)

	if m.opt.InsecureSkipVerify {
		dialer.TLSConfig = &tls.Config{
			InsecureSkipVerify: true,
		}
	}

	conn, err := dialer.Dial()
	if err != nil {
		return nil, errors.Wrap(err, "could not dial smtp server")
	}

	return conn, nil
}

func extractEmailDomain(email string) (string, error) {
	address, err := mail.ParseAddress(email)
	if err != nil {
		return "", errors.Wrapf(err, "could not parse email address '%s'", email)
	}

	addressParts := strings.SplitN(address.Address, "@", 2)
	if len(addressParts) != 2 { // nolint: gomnd
		return "", errors.WithStack(ErrUnexpectedEmailAddressFormat)
	}

	domain := addressParts[1]

	return domain, nil
}

func generateMessageID(domain string) string {
	// Based on https://www.jwz.org/doc/mid.html
	timestamp := strconv.FormatInt(time.Now().UnixNano(), 36)
	random := strconv.FormatInt(rand.Int63(), 36)
	return fmt.Sprintf("<%s.%s@%s>", timestamp, random, domain)
}